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PICList Thread
'[OT] CCDs'
1998\02\23@223916 by Herbert Graf

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       CCDs, how do they work? I am interested in hooking one up to my PC,
probably through a PIC but I have no idea on the basics. Do they work like
memory, give them an address and they give the data back, or is it more
complicated? (probably!) Anyways, I'm not looking at speed, just capturing
stills in astrophotography type applications, can't use the composite ones,
because well they are expensive and I would need a capture card, not easy on
a low end laptop! Any information would be usefull, even links on the
internet for info, but please no books, don't have the time to go out and
look for them. Thanks in advance. TTYL

1998\02\24@005241 by STEENKAMP [M.ING E&E]

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Hi,

>         CCDs, how do they work? I am interested in hooking one up to my PC,
> probably through a PIC but I have no idea on the basics. Do they work like
> memory, give them an address and they give the data back, or is it more
> complicated? (probably!) Anyways, I'm not looking at speed, just capturing
> stills in astrophotography type applications, can't use the composite ones,
> because well they are expensive and I would need a capture card, not easy on
> a low end laptop! Any information would be usefull, even links on the
> internet for info, but please no books, don't have the time to go out and
> look for them. Thanks in advance. TTYL

CCD's are basically a bunch of small capacitors on a string.  The
incoming light 'charges' the capacitors for a fixed time - called the
integration time.  The more intense the light, the more charge the
capacitor will accumelate during the integration period.  You
get the data out by shifting these charges out serially and converting
each to a digital value with an A/D.  You get two different
configurations - linear CCD's and matrix CCD's.  Linear CCD's are like
those used in fax machines and scanners.  It is a single line if cells.
The matrix CCD's are like those used in camcorders and is probably what
you are interested in.  Traditionally, the clocks needed by CCD's can be
quite exotic - multiple clocks with funny phase requirements and funny
voltages.  Newer CCD's have more relaxed clock requirements, but I
would still not suggest trying to drive a CCD directly.  The
manufacturer usually supplies a chipset which contains the clock
generater and timing generator for their CCD's.  These allow you to make
full use of special functions like shutter speed control.  The cheapest
and most readily obtainable matrix CCD's would be the ones used in
framegrabbers and CCTV cameras.  The resolution of these CCD's are
typical PAL or NTSC resolutions.  One I know of is a monochrome with
817x596 pixels (a viewable area of about 768x576).  So, as you can see,
the size of one image would be quite large.  Also, the format in which
these are clocked out, is optimized for generating interlaced composite
signals.  The even lines are first clocked out and then the uneven lines.
You do get CCD's that clock out all pixels in one go (progressive scan
CCD's) but they tend to be more expensive.  We use Sony CCD's - they have
a substantial range of matrix CCD's and support chips.  Texas Instruments
and Kodak also has some.

Niki

1998\02\24@053304 by Steve Baldwin

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>
>         CCDs, how do they work? I am interested in hooking one up to my
PC,
> probably through a PIC but I have no idea on the basics. Do they work
like
> memory, give them an address and they give the data back, or is it more
> complicated? (probably!) Anyways, I'm not looking at speed, just
capturing
> stills in astrophotography type applications, can't use the composite
ones,
> because well they are expensive and I would need a capture card, not easy
on
> a low end laptop! Any information would be usefull, even links on the
> internet for info, but please no books, don't have the time to go out and
> look for them. Thanks in advance. TTYL

Unfortunately the composite types are going to be cheaper for the same
number of pixels, simply because more people want camcorders than want to
do CCD astronomy. Also, anything they put in/with them to make life easier
for general use have a downside for astronomical use. Niki used the light
charging capacitor analogy, which is fine except that heat charges them
too. Not a problem if you are integrating for 20ms in a video camera but
when you are integrating for 30 seconds and looking for every photon you
can get, it's a real killer.
For this reason, astronomical CCDs are usually cooled with a heat pump (the
pros use liquid nitrogen).

The data (an analog value for each pixel) is clocked out using a 3 phase
clock. In a 'raw' CCD, these clocks need to have fast edges but the input
is hugely capacitive and needs to be driven over quite a few volts. So in
commercial (video) CCDs, many include the driver on the chip. For the
astronomers these are a source of heat and cause a bloom in the image.

The TI parts for low resolution and the Kodak parts for higher resolution,
seem to be the preferred types for astronomical use.

Probably the cheapest way to get a CCD onto a telescope is to buy a surplus
security camera and put tristate buffers in the clock lines. Disable them
while you integrate and then let the standard video circuitry clock the
information out. You'll still need a frame grabber and you'll need to sync
your integration start and stop with the framing. You're going to need a
frame grabber or fast A/D with any CCD solution.

Before you go down this road, don't forget one small aspect. Every frame of
a 640x480 image at 12 bits is half a meg of data. Where are you going to
put it ?

A standard video camera has sensitivity levels similar to your eyeball and
is pretty effective at prime focus. Like standard astrophotography though,
it shows just how good your mounting system is (or isn't).

You'll find some web addresses in the popular astronomy magazines. I think
SBIG have quite a lot of info.

Steve.

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1998\02\24@074555 by frak

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excuse my ignorance ppl, but what is a CDD?



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1998\02\24@130256 by Martin R. Green

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A CCD (not a CDD) is a Charge Coupled Device, sort of an electronic
bucket brigade.  The charge on a capacitive cell is shifted into the
adjacent cell on a clock transition.  At the end of the "brigade" the
charge on the last device is buffered and presented to the outside
world.  CCD's are used for audio delay lines, and as this thread
refers to, video detectors.  This works because the charge on a cell
can be modulated by light falling on it through a window on the chip,
then the charges on all cells are clocked out through the output
buffer.  When the entire image is clocked out, the image is captured
again and the process starts over for each frame.

CIAO - Martin.

On Tue, 24 Feb 1998 12:44:38 GMT, FRAK!FRAK!FRAK!
<MCHU5FANspamKILLspamFS2.EE.UMIST.AC.UK> wrote:

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1998\02\24@133908 by wwl

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>Before you go down this road, don't forget one small aspect. Every frame of
>a 640x480 image at 12 bits is half a meg of data. Where are you going to
>put it ?
Yeah, disk storage and RAM are soooo expensive these days  :-)

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1998\02\24@180623 by Robert Nansel

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Amateur Astronmers have been building and using CCD cameras for their
telescopes for five or six years. These are inexpensive units (less than
$300), _very_ sensitive (they use thermoelectric coolers & operate at -60
degrees F), and are eminently hackable.

One good source of info is _The CCD Camera Cookbook_, which gives the whole
scoop. The book is by Richard Berry, Veiko Kanto, & John Munger, and was
published by William-Bell, Inc. in 1994. You can reach the publisher at:

      PO Box 35025
      Richmond, VA 23235
      Toll-free credit card order phone: (800) 825 7827
      Information: (804) 320 7016
      Fax: (804) 272 5920

You can find out more about the whole deal on the Web at:

       http://wvi.com/~rberry/cookbook.htm

"Detailed instructions on how to build the Cookbook 211 and the Cookbook
245 cameras. Lots and lots of information packed into 176 pages. Includes
diskette with software to lead you through construction of the cameras step
by step, and image acquisition software. Requires IBM compatible PC.
$29.95."

Now, the chips they are using for this are TI's monochrome TC211 (192 by
165) and the TC245 (755 by 242). The TC245, besides having more pixels, is
fancier in its read circuitry. It is closely related to the TC244 color
CCD. According to the Cookbook, the TC244 is identical to the TC245 except
the TC244 has the RGB color mask coated on top of the silicon, and the
TC245 doesn't; the electronics is the same, though.

The (1994) single quantity prices quoted in the book are  around $32 for
the TC 211 and under $150 for the TC245. I know these prices are greater
than what NTSC color cameras cost, BUT you get exact control over the CCD
chip with software (the PC controls everything in these designs).

For example, one capability that these cameras have is _binning_, that is,
clocking two adjacent pixels into the same sense line, thus halving the
resolution & increasing sensitivity & speed. Or clocking out only a portion
of an image. Combine these two ideas and you can get realtime foveal vision
_without_ custom silicon.

--BN

1998\02\24@185357 by William Chops Westfield

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While we're talking about CCD cameras, does anyone know if any of the
cheap cameras out there can be synced in any sense to an external signal?
I want to combine several cameras as a cheap method of achieving high speed
(slow motion) imaging, by having each camera take its frames in between the
other cameras frames...

Thanks
bill W

1998\02\25@014003 by Peter van Hoof

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There is a problem doing a thing like this.
the cheaper camera's (connectix etc.) are quite insensitive to light,
the cap's in the ccd's charge / discharge slow under influence of light,
this gets a little better if there is ample light available but
still causes fuzzy images when trying to get an image from
moving targets. I have no idea if there would be any camera's
like this that synch to external signals but cannot believe you would find
such an option readily available on a cheep one.

Peter


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1998\02\25@131909 by Lee Jones

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> While we're talking about CCD cameras, does anyone know if any of the
> cheap cameras out there can be synced in any sense to an external signal?

Probably not easily.  You could use whatever "frame ready"
timing signal the camera generates as feedback to control
a variable oscilator which replaces the camera's clock.
The effort to retrofit this might exceed the work to build
the control circuitry from scratch.

> I want to combine several cameras as a cheap method of achieving high speed
> (slow motion) imaging, by having each camera take its frames in between the
> other cameras frames...

If you have 20 cameras, you could get 20 times the frame
rate.  But I assume the subject is in motion.  If so, the
images will probably be blurred since each camera's
integration time will be unchanged.  The image each CCD
builds up will be 20 "frames" worth.

You might be able to get around this if the camera module
has automatic exposure by using 20 times the light level.
But the camera module probably has a minimum integration
time (that I don't recall being a readily available spec
for the low end units).
                                               Lee Jones

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